1. Field of the Invention
The present invention relates to a Francis-type wheel of a hydraulic machine equipped with a tip-forming member which in the main extends the crown of the wheel. The invention also relates to a method of reducing the fluctuations in a flow interacting with such a wheel.
2. Brief Description of the Related Art
Previously disclosed in the field of hydraulic turbines, in particular Francis-type turbines, is the equipment of the downstream region of the crown of a wheel with an axisymmetric member, often referred to as a “tip”, the external surface of which substantially extends the wetted surface of the crown of the wheel having a diameter which may reduce in the downstream direction. This member or “tip” extends the guiding of the flow into proximity with the axis of rotation of the wheel. Similarly previously disclosed is the use of guiding members or tips in turbine pumps and other paddle turbines.
In previously disclosed hydraulic turbines, the flow crossing the wheel may give rise to eddies or turbulence phenomena, commonly referred to as “torches”, which, under certain loadings, are in the form of a “corkscrew”. These turbulence phenomena in the form of a torch are troublesome to the extent that they cause fluctuations in pressure and/or power which can adversely affect the stability of the network that is fed by the machine as well as the mechanical outputs of this machine.
Previously disclosed in WO-A-2005/038243 is the use of a tip comprising two surfaces, respectively convergent and divergent in the direction of an axis of rotation of the wheel, which permits the turbulence phenomena to be limited to a large degree. Turbulence phenomena remain present at certain speeds, however, and the present invention proposes an alternative solution permitting these turbulence phenomena to be eliminated or greatly reduced.
Also previously disclosed in U.S. Pat. No. 2,758,815 is the arrangement of small-diameter bores in the form of a tip to permit recirculation, inside the tip of a Francis-type wheel, from the bottom towards the top, for a Francis-type wheel having a vertical axis. A quantity of water exits from the tip via these openings in such a way as to constitute jets deflected towards the downstream direction by the principal flow passing though the turbine in order to form around the tip a mixed flow zone which constitutes a current ring surrounding the meridian region of the tip. This type of function is not always effective to the extent that the “pumping” effect due to the tip cannot be guaranteed.
Also previously disclosed in FR-A-1 162 872 is the arrangement of a cylindrical tube below a tip of a conical wheel without knowing precisely how the water circulates inside and around this tube. This tube is intended to guide the water after its passage inside the wheel, in such a way that it is caused to extend for a considerable height inside the suction conduit of the installation. The tube limits the output volume of the principal flow as it leaves the wheel, which increases the speed of this flow and, as a consequence, the output losses. Ribs connect the tube mechanically to the conical tip, but without their influence on the path of the flow being quantified. In addition, the tube suspended at the tip substantially increases the overall size of the wheel, which presents difficulties for the installation of the wheel at its place of use.